The present invention relates to a process for preparing thermoplastic polyimides. More particularly, the invention relates to a process for preparing certain crystalline or semi-crystalline, chemically resistant polyimides which are not easily produced by conventional procedures.
The thermoplastic polyimides with which the present invention is concerned are high performance thermoplastics. Because of the thermoplastic nature of these polymers, they can be formed into useful articles by such techniques as injection molding, extrusion, blow-molding and thermoforming. These polymers are characterized by high heat distortion temperatures and excellent physical properties.
In general, polyimidse are prepared by reacting an aromatic dianhydride with an organic diamine. In the search for polymers having superior performance properties, certain polyimides have been discovered which have crystalline or semi-crystalline characteristics and good resistance to chemicals, such as cleaning solvents, fuels and oils, and the like. These crystalline or semi-crystalline polyimides often have exceptionally high glass transition temperatures, which make them useful for high temperature applications. In general, these polyimides have linear, rigid aromatic moieties in the aromatic dianhydride residues or the organic diamine residues of the polymer backbone. Polyimides having these structures have posed difficulties in their manufacture, primarily because of their relative insolubility in organic solvents.
Thermoplastic polyimides, such as polyetherimides, have been prepared by a variety of processes. The two basic processes used for making these polymers are the so-called "melt polymerization" process and the "solution polymerization" process. The melt polymerization process has been described in several U.S. patents, representative of which is U.S. Pat. No. 3,803,085 to T. Takekoshi and J. Kochanowski. This process involves combining an aromatic dianhydride and an organic diamine and heating the mixture under an inert atmosphere to form a homogeneous melt. Water formed during the polymerization reaction is removed at a temperature of up to 350.degree. C., and the final stage of the reaction is advantageously conducted under reduced pressure to facilitate removal of water. The basic melt polymerization technique has been improved by employing certain catalysts to enhance yields or reaction rates. (E.g., see Takekohi et al, U.S. Pat. No. 3,833,544, F. Williams III et al., U.S. Pat. No. 3,998,840 and Takekoshi, U.S. Pat. No. 4,324,882.) In addition, the melt polymerization method has been adapted to the continuous mode by conducting the reaction in an extrusion apparatus. (E.g., see Takekoshi et al., U.S. Pat. No. 4,011,198 and Banucci, et al., U.S. Pat. No. 4,073,773.)
Solution polymerization is generally conducted by reacting an aromatic dianhydride and an organic diamine in an inert solvent at temperatures up to about 200.degree. C. With this procedure, water formed during the reaction is typically removed by azeotropic distillation. The resulting polymer is generally recovered by mixing the reactant solution with a precipitant, such as methanol. The reaction solvents employed for solution polymerization reactions are selected for their solvent properties and their compatability with the reactants and products. High-boiling, nonpolar organic solvents have been preferred. (E.g., see Takekoshi, et al., U.S. Pat. No. 3,991,004.) Dipolar, aprotic solvents and phenolic solvents have also been used. (E.g., see Takekoshi, et al., U.S. Pat. No. 3,905,942.)
The melt polymerization and solution polymerization techniques suffer from certain disadvantages. The melt polymerization technique involves combining monomers which have widely differing volatilities at the high temperatures employed. Because of the disparate volatilities of these components, controlling the stoichiometry of the mixture has been proven difficult. A further disadvantage of the melt polymerization procedure is that the reaction mixture passes through a so-called "cement stage" as polyamide acid intermediate is formed. During this phase of the reaction, the reaction mixture becomes very viscous and difficult to process. The solution process, on the other hand, permits accurate control of stoichiometry, but reaction times are relatively long and it is sometimes difficult to achieve complete conversion of the reactants or intermediate polyamide acids to the polyetherimide product.
Because of these disadvantages, several processes have been developed which combine the two techniques. For example, Takekoshi, U.S. Pat. No. 4,221,897 describes reacting an aromatic dianhydride and an organic diamine in an aqueous reaction medium substantially devoid of organic solvent. This reaction produces a polyamide acid intermediate which is recovered as a finely divided powder which can be used to make high molecular weight polyimide by melt extrusion. In a similar process, Banucci et al. describe in U.S. Pats. Nos. 4,098,800 and 4,197,396, a process which involves reacting an aromatic dianhydride and an organic diamine in an inert organic liquid selected from methylene chloride, chloroform, 1,2-dichloroethane and mixtures thereof with acetone. The reaction produces an oligomeric polyamide acid which is substantially insoluble in the organic liquid and thus separates from the reaction mixture as a precipitate. The polyamide acid may be recovered in powdered form which is useful in powder-coating procedures wherein the desired polyetherimide is obtained in situ by heating to a temperature above the glass transition temperature. In U.S. Pat. No. 4,417,044, S. L. Parekh discloses a process for making polyimides which involves reacting an aromatic dianhydride with an organic diamine in an inert solvent to form a prepolymer-solvent mixture, removing the solvent from the mixture by thin-film evaporation and heating the resulting prepolymer (e.g., in an extruder) to a temperature above its glass transition temperature to form the desired polyimide product.
Although some of the foregoing procedures have been found very useful for preparing conventional polyetherimides, they have not been entirely satisfactory for the preparation of the crystalline, solvent resistant polyimides containing linear, rigid structural units. For example, it has been found that polyetherimides prepared from an aromatic bis(etheranhydride) and a major proportion of p-phenylene diamine have crystalline characteristics and higher temperature and chemical resistance than corresponding polyetherimides prepared from m-phenylene diamine. When such a polyetherimide is produced by first isolating the polyamide acid intermediate, it has been found that a substantial amount of the diamine is bound to the polyamide acid through relatively labile ionic bonds. When these materials are extruded at elevated temperatures, the ionic bond is broken and a significant amount of the diamine is lost through volatilization. This volatilization not only makes controlling the stoichiometry difficult, but also poses a significant health hazard, since the diamine is lost to the atmosphere and condenses on equipment and surfaces surrounding the extruder.
A need exists for a straightforward, efficient process for preparing high performance polyimides. Such process should permit accurate control of the stoichiometry, utilize reasonable reaction times and conventional equipment.